The striker, who signed from Genk in the summer, missed a free header from a Gabriel Agbonlahor cross in the second half, raising questions over his selection ahead of proven goalscorer Darren Bent.
"That was a big moment," Lambert said. "When you have chances like that you have to put them away.
"Darren's threat is going in behind. Christian gives you a different option. That was the only reason we did that. Darren has been fine about it."